1. Field of the Invention
The invention relates to the field of the supply of electrical power to light sources. In particular, the invention relates to a system and method for controlling the supply of electrical power to and for thermal management of light sources and more precisely to light-emitting diodes (LEDs). The invention also relates to a luminous device for an automotive vehicle.
2. Description of the Related Art
A light-emitting diode (LED) is an electronic component capable of emitting light when an electrical current passes therethrough. The light intensity emitted by an LED is in general dependent on the magnitude of the electrical current passing therethrough. Inter-alia, an LED is characterized by a current magnitude threshold value. This maximum forward current generally decreases with increasing temperature. Likewise, when an LED is emitting light, a voltage drop equal to its forward voltage is observed across its terminals, which drop in general also decreases with increasing temperature. In the automotive field, recourse is increasingly being made to LED technology for various luminous signalling solutions. LEDs are used in order to provide luminous functions such as daytime running lights, signalling lights, etc. Whereas they are exposed to high operating temperatures, just after an automotive vehicle has been started up they operate at the ambient temperature of the vehicle. This temperature may especially reach negative temperatures.
It is known in the art to use a control circuit to control the supply of power to an assembly or group of LEDs. The circuit defines the voltage applied to a load arm comprising the group of LEDs connected in series. In the field of luminous devices for automotive vehicles, it is particularly important to be able to provide a constant luminosity in order to guarantee the safety of users of the vehicle and of other road users. In order to deliver a constant supply of power, known control circuits use various types of DC/DC converters to convert the DC voltage delivered for example by a car battery into a DC load voltage that is dependent on the number of LEDs to be supplied with power. The forward voltage of the LEDs is higher at low temperatures than at an ambient temperature of 25° C. or at the operating temperature of the LEDs. In general, a DC/DC converter is however dimensioned depending on the characteristics of the supplied LEDs at their operating temperature. Therefore, the situation may arise where the converter is not capable of delivering an electrical voltage level that is high enough to supply the load LEDs, when the latter have been cooled. This defect may be limited to a few volts.
It is known to approach this problem by dividing the assembly of LEDs supplied by the DC/DC converter into a plurality of independent arms, each arm comprising a dedicated current source. This solution is recommended if a large number of LEDs is to be supplied, and if a single arm containing all of the LEDs in series would lead to an overall forward voltage clearly higher than the maximum voltage that the converter can deliver. However, this solution is not suitable when the overall forward voltage only slightly exceeds at low temperatures the maximum voltage that the converter can deliver. Specifically, this solution consumes a lot of space on a printed circuit board and uses a lot of additional electrical components—at least one current source per arm.
Another known alternative is to share the power supply of a plurality of independent arms of LEDs over time. In the case of this time sharing, the DC/DC converter supplies each of the arms alternatively. This solution is feasible when a large number of LEDs is to be supplied, and when a single arm containing all of the LEDs in series would lead to an overall forward voltage clearly higher than the maximum voltage that the converter can deliver. However, this solution is also unnecessarily elaborate when the overall forward voltage only slightly exceeds at low temperatures the maximum voltage that the converter can deliver. Specifically, this solution requires a more complex and expensive power supply device in order to supply in alternation the various arms of LEDs.